<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
    <style>
      body {
        margin: 0;
        padding: 0;
        background-image: url(../images/grid.png);
      }
      canvas {
        background-color: rgba(255, 0, 0, 0.1);
      }
    </style>
  </head>
  <body>
    <canvas id="tutorial" width="300" height="300">
      你的浏览器不兼容Canvas,请升级您的浏览器!
    </canvas>

    <script>
      window.onload = function () {
        let canvasEl = document.getElementById('tutorial')
        if (!canvasEl.getContext) {
          return
        }
        let ctx = canvasEl.getContext('2d') // 2d | webgl

        // 1.准备一张图片
        var image = new Image()
        image.src = '../images/backdrop.png'

        image.onload = function () {
          // 2.开始用Canvas来绘制图片
          ctx.drawImage(image, 0, 0, 180, 130)

          // 3.绘制折线
          ctx.beginPath()
          ctx.moveTo(40, 100)
          ctx.lineTo(50, 70)
          ctx.lineTo(60, 90)
          ctx.lineTo(100, 30)
          ctx.lineTo(170, 90)
          ctx.stroke()
        }
      }
    </script>
  </body>
</html>
